The present invention relates to a control architecture for asynchronous transfer mode (ATM) networks, and specifically relates to a burst-level control for controlling access in an ATM network.
While the following description describes the invention in conjunction with a LAN, the invention is not so limited and is applicable to any ATM network.
There is tremendous interest in the development of ATM based LANs that provide connectivity between local work-stations and servers and are also suitable for use as a hub.
Current LAN technologies have media access control which ensures that once a station gains access to the medium, no frame loss results (except due to random noise). Many ATM-based switch architectures, on the other hand, do not have such control. Moreover, because of the limited amount of high speed memory that is provided in a switch, it is possible for there to be significant cell loss, especially when there is hot spot traffic from several bursty sources all directed to a single output port. For example, such a condition occurs in a client server model. The resulting cell loss leads to large frame loss when compared to systems having media access control. Therefore, it is essential that during periods of overload the bandwidth at a hot-spot output port be shared in a manner similar to that in a shared medium network.
A solution to implementing media access control and shared bandwidth at a hot-spot output port is to institute a burst-level control that manages media access in an ATM LAN. Burst level control is preferable in an ATM LAN when the following conditions are met: sources generate large bursts (compared to the amount of buffer memory in the switch) that will result in buffer overflows in the absence of any control; blocking a burst at the beginning of the burst is preferable to having retransmissions of bursts already in progress and frequent retransmissions due to cell loss increases the effective load on the system, resulting in an end-to-end throughput that is several times slower than that of a shared medium network.
However, it is important that the burst level control be done in real time so that the latencies in admitting new bursts do not become a significant bottleneck.
The present invention describes a burst level control method and apparatus for use in an ATM LAN. The burst level provides both a media access control and a fast and efficient call admission control.
By using such a burst level control, not only can ATM LAN performance be comparable to the performance of media access based technologies, e.g., FDDI, with respect to hot spot traffic, but the ATM LAN is able to provide a total bandwidth that is N times the bandwidth of a shared media system (where N is the number of ports). Moreover, the burst level control is scalable to wide area networks because the control does not rely on reactive mechanisms as a primary method of congestion control and is a small add-on function that is useful for Public Networks.